I guess the meaning of supersonic has changed since I went to school.
Agreed, I think that supersonic speed is literally that at which the speed of sound is exceeded, and is thus relative to the surrounding air. The jet stream wind speed would have to be subtracted from the ground speed to give the actual air speed of the aircraft.
As well, the speed of sound (Mach 1) is c. 760 mph @ STP (standard temperature and pressure, 20 C @ sea level) but decreases with altitude, as atmospheric pressure drops. As atmospheric pressure drops, the air becomes less dense, and sound travels more slowly. This chart helps:https://www.fighter-planes.com/jetmach1.htm
Per the chart, the speed of sound is about 660 mph at typical airliner altitudes. The aircraft under consideration is said to have hit a maximum ground speed of 825 mph, but if we subtract the 260 mph tailwinds, we're left with a maximum airspeed of 565 mph, well below supersonic (660 mph) at that altitude.
To put it another way, if you could somehow have projected a loud sound from the aircraft, its waves would have traveled in front of (that is, faster than) the aircraft, not behind the aircraft.